Smith has gone from a defenseman to a forward and back to a defenseman since entering the NHL during the 2011-12 season.

The Detroit Red Wings selected the native of Mimico, Ontario, in the first round (No. 27) of the 2007 NHL Draft. He attended the University of Wisconsin for three seasons before turning pro and signing with the Red Wings on May 27, 2010. He played all of the 2010-11 season with Grand Rapids of the American Hockey League and began 2011-12 there but was called up after the Red Wings traded defenseman Mike Commodore and made his NHL debut against the San Jose Sharks on Nov. 27, 2011. He scored his first NHL goal against the Minnesota Wild on March 2, 2012.

Smith became an NHL regular during the 2012-13 season and had NHL career highs in goals (five) and points (19) in 2013-14. He remained with the Red Wings until Feb. 28, 2017, when Detroit traded him to the New York Rangers. He signed a four-year contract with the Rangers on June 29, 2017.

However, the Rangers struggled during the 2017-18 season. Smith lost his regular spot on defense and was sent to Hartford of the AHL in February 2018. Soon after, he broke his hand, ending his season.

But Smith revived his career under new coach David Quinn. He reported to training camp in the fall of 2018 in top shape and earned a role as a fourth-line forward who could drop back to become a defenseman on the penalty kill. He ended up playing 63 games and finished with 13 points (four goals, nine assists) on a rebuilding team.

He began 2019-20 playing mostly as a forward but became a full-time defenseman again late in the season after Brady Skjei was traded on Feb. 24, 2020.

An unrestricted free agent, Smith agreed to a one-year contract with the Carolina Hurricanes on July 29, 2021. He had eight points (four goals, four assists) in 45 regular-season games and four points (one goal, three assists) in 14 Stanley Cup Playoff games before signing a two-year, $2.2 million contract with the New Jersey Devils on July 13, 2022.

Smith signed a one-year, $1 million contract with the Dallas Stars on July 1, 2024, and a one-year, two-way contract with the Columbus Blue Jackets on Nov. 24, 2025.